Synthesis and crystal structure of (NH4)[Ni3(HAsO4)(AsO4)(OH)2]

The crystal structure of the title compound consists of 2 ∞[Ni3As2(OH)6/3O18/3O1/1(OH)1/1] layers extending parallel to (001) and exhibits disorder of the (O/OH) units of the (hydrogen)arsenate anion; the ammonium counter-cations are sandwiched between adjacent layers.

The title compound, ammonium trinickel(II) hydrogen arsenate arsenate dihydroxide, was synthesized under hydrothermal conditions.Its crystal structure is isotypic with that of K[Cu 3 (HAsO 4 )(AsO 4 )(OH) 2 ] and is characterized by pseudo-hexagonal (001) 2  1 [Ni 3 As 2 O 18/3 (OH) 6/3 O 1/1 (OH) 1/1 ] À layers formed from vertex-and edge-sharing [NiO 4 (OH) 2 ] octahedra and [AsO 3.5 (OH) 0.5 ] tetrahedra as the building units.The hydrogen atom of the OH group shows occupational disorder and was refined with a site occupation factor of 1/2, indicating the equal presence of [HAsO 4 ] 2-and [AsO 4 ] 3-groups.Strong asymmetric hydrogen bonds between symmetry-related (O,OH) groups of the arsenate units [O� � �O = 2.588 (18) A ˚] as well as hydrogen bonds accepted by these (O,OH) groups from OH groups bonded to the Ni II atoms [O� � �O = 2.848 (12) A ˚] link adjacent layers.Additional consolidation of the packing is achieved through N-H� � �O hydrogen bonds from the ammonium ion, which is sandwiched between adjacent layers [N� � �O = 2.930 (7) A ˚] although the H atoms could not be located in the present study.The presence of the pseudohexagonal 2 1 [Ni 3 As 2 O 18/3 (OH) 6/3 O 1/1 (OH) 1/1 ] À layers may be the reason for the systematic threefold twinning of (NH 4 )[Ni 3 (HAsO 4 )(AsO 4 )(OH) 2 ] crystals.Significant overlaps of the reflections of the respective twin domains complicated the structure solution and refinement.

Chemical context
The natural occurrence of numerous arsenates creates a mineralogical spotlight and hence the need for a crystalchemical classification of the respective minerals (Drahota & Filippi, 2009;Majzlan et al., 2014).However, arsenate(V) minerals or synthetic compounds have been investigated not only because of their rich structural chemistry but also for their technical relevance, for example in terms of non-linear optical properties (Dhouib et al., 2014(Dhouib et al., , 2017) ) or protonic conductivity (Chouchene et al., 2017a,b).
During experiments targeted at the incorporation of arsenate(III) or -(V) anions into transition-metal oxido-tellurates(IV), the title compound was obtained serendipitously under hydrothermal conditions.In the present paper, we report on the synthesis and crystal structure analysis of (NH 4 )[Ni 3 (HAsO 4 )(AsO 4 )(OH) 2 ], (I), and its comparison with related crystal structures.

Structural commentary
The asymmetric unit of (I) comprises two Ni, one As, four O, one N and two H atoms; the atoms belonging to the ammonium cation could not be localized.Except for O3, which is situated on a general position (multiplicity 8, Wyckoff letter j), all the other atoms are at special positions of space group C2/m: Ni1 (2a) and N1 (2c) exhibit site symmetry 2/m, Ni2 (4e) site symmetry 1, and the remaining atoms site symmetry m (4i).
Both Ni II atoms are surrounded by six oxygen atoms, two of them (O4 and its symmetry-related counterpart) being parts of hydroxide groups.The [NiO 4 (OH) 2 ] units have a distorted octahedral shape with the hydroxide groups being in trans positions (Fig. 1).The OH groups have the shortest Ni-O bond lengths in both coordination polyhedra.The [NiO 4 (OH) 2 ] octahedra are connected to four neighboring units, all by sharing edges to form 2 1 [Ni 3 (OH) 6/3 O 12/2 ] 8-layers extending parallel to (001) (Fig. 2).The O atoms and OH groups of these layers form a hexagonal close packed (hcp) like arrangement where 3/4 of the voids are filled with Ni II atoms and 1/4 of the voids, corresponding to the Wyckoff 2a site, being vacant.If this void was also occupied by an Ni II atom, the resulting layer resembles that present in the simple C6 CdI 2 structure [also referred to as the brucite [Mg(OH) 2 ] structure; Wells, 1975].In the title compound, the As atoms are located above and below each void in the 2 1 [Ni 3 (OH) 6/3 O 12/2 ] 8-layer, sharing three oxygen atoms with the layer on either side.The bond-valence sums (BVS; Brown, 2002) of the nickel atoms were determined to be 2.06 (Ni1) and 1.99 (Ni2) valence units (v.u.) based on the parameters of Brese & O' Keeffe (1991), in good agreement with the expected value of 2.00.
The As V atom in (I) is tetrahedrally coordinated by oxygen atoms, spanning a range from 1.679 (10) to 1.701 (6) A ˚. Contrary to expectations (Schwendtner & Kolitsch, 2019), the longest As-O bond is not associated with the OH functionality (O2), which instead shows the shortest of all As-O bonds and is located at the apex of the tetrahedron pointing away from the hexagonal layer.Apart from the bonded H atom, O2 solely belongs to the arsenate group and is not shared with other building blocks.The average As-O bond length is 1.694 (10) A ˚for the resulting [AsO 3.5 (OH) 0.5 ] unit (Table 1), which is comparable to the mean As-O bond length of 1.687 ( 26 Atomic environments of the Ni II and As V atoms in the crystal structure of (NH 4 )[Ni 3 (HAsO 4 )(AsO 4 )(OH) 2 ].Displacement ellipsoids are drawn at the 90% probability level.Symmetry codes refer to Table 1.

Figure 2
The crystal structure of (NH 4 )[Ni 3 (HAsO 4 )(AsO 4 )(OH) 2 ] projected on the ( 001 For the ammonium cation associated with the N1 site, no hydrogen atoms could be located.The closest oxygen atoms for hydrogen-bonding interactions are situated at distances of 2.930 (7) A ˚(4�), 3.008 (9) A ˚(2�) and 3.229 (4) A ˚(4�), respectively, which would correspond to hydrogen bonds of medium to weak strength.The site symmetry (2/m) of the N1 atom and the high number of possible acceptor sites for hydrogen-bonding make it most likely that the tetrahedral [NH 4 ] + cation is orientationally disordered, which complicates the localization of its hydrogen atoms.

Synthesis and crystallization
The solid starting materials, NiO (0.1490 g; 1.99 mmol), TeO 2 (0.1596 g; 1.00 mmol) and As 2 O 3 (0.1974 g; 1.00 mmol), were manually mixed in a small Teflon container with an inner volume of ca 4 ml.Then, 0.49 g NH 3 (aq), 25% wt (7.2 mmol) and subsequently demineralized water were added to obtain a final filling degree of ca 3/4.The mixture was manually stirred before the container was closed with a Teflon lid.The sealed container was heated inside a steel autoclave under autogenous pressure at 483 K for one week.After cooling down to room temperature within 3 h, a grayish green solid had formed, which was filtered off and dried overnight.The reaction product was identified by powder X-ray diffraction as a mixture of elemental tellurium (' 50% wt ; Bradley, 1924), responsible for the gray color, (NH 4 )[Ni 3 (HAsO 4 )(AsO 4 )-(OH) 2 ] (' 45% wt ) and small amounts of (NH 4 )H 2 AsO 4 (' 5% wt ;Delain, 1958).This indicated that a redox reaction between the As III and Te IV starting materials had occurred, yielding As V and elemental tellurium: In a subsequent re-synthesis, the title compound was obtained with higher yields when Ni(NO 3 ) 2 (H 2 O) 6 (0.3546 g; 1.08 mmol), As 2 O 5 (H 2 O) x (x = 2-3; 0.1272 g; '0.46 mmol) and 0.76 g NH 3 (aq), 25% wt (10.5 mmol) were reacted under the same hydrothermal conditions.However, the obtained material still was not single-phase; the remaining reflections could not be assigned to any literature phase, indicating another unknown phase or even phases.Crystals of (I) have the form of light-green blocks with sharp edges.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.
All investigated crystals were systematically twinned with three domains present that are related by a 120 � rotation around the c* axis (Fig. 4).The ratio of a and b (1.721) in the C-centered unit-cell is very close to ffi ffi ffi 3 p and underlines the relation to pseudo-hexagonality.The reflections of the corresponding domains were rather diffuse for many crystals, which resulted in the necessity of testing many crystals until one suitable for the final diffraction experiments was found.Moreover, since a significant overlap of neighboring reflections of different domains occurred frequently, the final measurement was performed with an increased sample-todetector distance of 100 mm.Integration was attempted based either on only the most intense domain or all three domains simultaneously.The intensity data of the one-domain integration led to lower reliability factors in the resulting refinement compared to the three-domain approach (R 1 = 0.050 versus 0.068).However, due to the overlap of reflections, disregarding the other two domains during integration led to artifacts in the resulting refinement.These features were indicated by significant difference electronic-density peaks corresponding to the positions of the heavy atoms As and Ni in the other twin domains, which resulted in a cross-shaped pattern in the difference-Fourier plots (Fig. 5a).The corresponding three-domain integration shows significantly lower difference electron densities (Fig. 5b).Despite the higher resulting reliability factors, the data resulting from the threedomain integration was chosen for the final structure refinement.The ratios of the three twin domains refined to values of 0.653 (4):0.264(4):0.093(2).The CIF resulting from the onedomain integration can be found in the electronic supplementary information (ESI) for this article.Computer programs: X-AREA (Stoe, 2021), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b), DIAMOND (Brandenburg, 2016) and publCIF (Westrip, 2010).SHELXL (Sheldrick, 2015b) and with U iso (H) = 1.5U eq (O).Atom labels and coordinates were assigned in accordance with isotypic K[Cu 3 (HAsO 4 )(AsO 4 )(OH) 2 ] (Effenberger, 1989).
The only atom breaking the C2/m symmetry is atom H1 (under assumption of full occupancy).Because adjacent H1 sites are symmetrically connected by the 2 010 axis, it was attempted to resolve the disorder of H1 by a symmetry reduction to Cm with inclusion of the removed symmetry operation as the twin law.In the lower-symmetric space group, the As1 and O2 sites are both split into two positions.Extensive modeling attempts in space group Cm based on both one-domain and three-domain integrations were performed, but the disorder of the H1 atom could not be resolved on basis of the two data sets.In general, the Cm models were of inferior quality due to over-parametrization and strong correlations between atom pairs, resulting in significantly larger standard uncertainties of atomic coordinates, interatomic distances and negative displacement parameters for some atoms.Hence, C2/m was chosen as the space group of the final model, assuming an equal distribution of O and OH at the O2 site.
Single crystals of (NH 4 )[Ni 3 (HAsO 4 )(AsO 4 )(OH) 2 ] were investigated at both room temperature and 100 K, but no ordering of the hydrogen atoms was observed at the lower temperature.
In the final model (three-domain integration), the highest remaining positive and negative electron density peaks are located at the Ni1 site and 1.77A ˚from O1, respectively.
Figure 1 ) plane (a) and viewed along [010] (b).Ni II atoms are drawn as blue, As V atoms as green, O atoms as red, N atoms as purple and H atoms as white spheres with arbitrary radius.Hydrogen bonds are drawn as orange dashed lines.position as well [O4� � �O2 = 2.848 (12) A ˚].The remaining interspace is occupied by the [NH 4 ] + cation (Fig. 2).

Table 2
Experimental details.